The present invention relates to a channel level adjusting apparatus well suited for use with the head end of a community television receiving equipment.
Generally, the field intensity of the broadcast waves arriving at the community receiving antenna of a community receiving equipment differ for different channels. As a result, the television signals of different channels received by the antenna are each subjected to level adjustment so that the amplitude levels of the television signals of all the channels are adjusted to a fixed value and the signals are then fed to the individual subscriber terminals. In this case, there is less chance of a situation arising in which the input amplitude level becomes unequal for all the different channels, and a situation frequently occurs in which the input amplitude level of a particular smaller number of channels is much higher than that of the other channels, as for example, the input amplitude level for a plurality of channels of wide coverage stations is uniform and the input amplitude level of only one or two local stations differ greatly from that of the former.
With a known channel level adjusting apparatus of this type in which the unequal input amplitude levels of the television signals on the respective channels are adjusted to generate the television signals of the equal amplitude level for all the channels, it has been a usual practice so that the television signals of all the channels received by the antenna are separated for the respective channels by the bandpass filters which are each provided for one of the channels, are subjected to level adjustment for each channel or particular channels, are again subjected to level adjustment with respect to the television signals of all the channels and are then generated. This type of channel level adjusting apparatus employing a bandpass filter for each channel is disadvantageous in that where the number of channels is large, the same number of bandpass filters are required and moreover it is much wasteful in that the television signals are separated for all the channels by the bandpass filters despite the fact that as mentioned previously a situation occurs practically at all times in which the number of channels differring in input amplitude level and requiring level adjustment is small.
Another type of channel level adjustment apparatus having bandpass filters which are not the same but smaller in number than the channels is known in the art, which comprises a bandpass filter for passing only the television signal of that channel which is to be subjected to level adjustment and one or two broad-band bandpass filters for passing the television signals of the remaining channels. This type of apparatus is also disadvantageous in that the filter characteristic of the broad-band bandpass filter is not sharp thus tending to cause interference with the signals of the other bandpass filters and that if a notch filter of a sharp cutoff characteristic is connected to the broad-band bandpass filter to attenuate the undesired frequency components and thereby to prevent the interference, the sharp cutoff characteristic tends to cause the cutoff frequency to change with a change in temperature and thereby to make it impossible to ensure stable performance.